1. Field of the Invention
The present invention relates to an optical device, and in particular, to an optical filter.
2. Description of the Related Art
Spectrometers are widely used for measuring spectra of test samples (for example, liquid crystal displays) and calculating optical parameters such as chromaticity and luminance. A conventional spectrometer sequentially comprises a grating, which is generally used for a beam-splitting device, a long-pass filter, which is also called an order-sorting filter, disposed in front of a photodetector. The order-sorting filter is used to filter a beam of second order diffracted light having a shorter wavelength spatially overlapped with a beam of first order diffracted light having a longer wavelength. The conventional order-sorting filter has an optical film partially coated thereon, resulting in a sharp step-liked edge of the optical film between a coating region and a non-coating region of the order-sorting filter. The sharp step-liked edge of the optical film does not allow light signals to be received by the photodetector behind the filter because an incident light beam can not transmit through the sharp step-liked edge of the conventional order-sorting filter and thus a spectrum with a dark line occurs. The dark line spectrum problem causes spectra measuring error, a reduced measuring area of the order-sorting filter, a reduced velocity and less accuracy of in-line measurement.
U.S. Pat. No. 6,700,690 describes a conventional variable tunable bandpass optical filter 10 comprising a filter pair 1 and 2 as shown in FIG. 1. The conventional variable tunable bandpass optical filter 10 is comprised of two transparent optical substrates, upon each of which is deposited a linearly variable multilayered interference filter coating which varies in total thickness from end to end in a wedged fashion, creating a filter pair 1 and 2. The wedged coating imparts a corresponding variation of the center wavelength of the filter bandpass with respect to the linear position of the filter pair 1 and 2. The variable tunable bandpass optical filter 10, referred to as a linear variable filter (LVF), can be adjusted to a specific center wavelength by moving the filter pair 1 and 2 together linearly through the incident beam. The size of the conventional variable tunable bandpass optical filter 10, however, is too large (length×height×width: 57 mm×10 mm×0.7 mm) to install in a small-sized spectrometer. The conventional variable tunable bandpass optical filter also has shortcomings such as a difficult and costly fabricating process.
Therefore, an optical device with quick and precise spectra measurement is needed.